Coin-shaped lithium battery

ABSTRACT

A coin-shaped lithium battery is disclosed. The coin-shaped lithium battery comprises a battery can containing therein an anode pellet composed of lithium or lithium alloy and a cathode pellet which are arranged to face each other with a separator provided between the pellets. At least one of the anode pellet and the cathode pellet has its central part swollen to form a curved surface. The battery can is elastically deformed to be aligned with the curved surface. In the coin-shaped lithium battery, total height of the anode pellet and the cathode pellet in outer rim part is smaller than total height in central part by 4 to 12%.

This application is a continuation of application Ser. No. 08/361,662,filed Dec. 22, 1994 and now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a coin-shaped lithium battery constituted by abattery can containing therein an anode pellet composed of lithium orlithium alloy and a cathode pellet which are arranged to face each otherwith a separator provided between them.

In the coin-shaped lithium battery constituted by a battery cancontaining therein an anode pellet composed of lithium or lithium alloyand a cathode pellet which are arranged to face each other with aseparator provided between them, such as a coin-shaped lithium manganesebattery, the anode material, that is, the anode pellet composed oflithium or lithium alloy decreases in volume, and the cathode material,that is, the cathode pellet increases in volume as the batterydischarges. The decrease in height of the anode pellet and the increasein height of the cathode pellet are not equal, reducing the total heightof the anode pellet and the cathode pellet. Consequently, contact of thebattery can, the anode pellet and the cathode pellet with one another,that is, contact between anode and cathode cannot be maintainedsatisfactorily in the latter half of discharge, and hence theunsatisfactory contact causes a significant increase in internalpressure and discharge failure.

In a conventional coin-shaped lithium manganese battery constituted by abattery can 44 containing therein parallel disc-shaped anode pellet 41and cathode pellet 42 facing each other with a separator 43 betweenthem, an anode cap 45 and a cathode can 46 forming the battery can 44are caulked with an insulation sealing gasket 47 provided between them,and the anode cap 45 is elastically deformed, as shown in FIG. 1. In thecoin-shaped lithium manganese battery of this structure, hereinafterreferred to as Conventional Example 1, the decrease in height of theactive material in the course of discharge causes the anode cap 45 toelastically deform, so that satisfactory contact between anode andcathode is maintained.

In another conventional coin-shaped lithium manganese batteryconstituted by a battery can 54 containing therein an anode pellet 51and a cathode pellet 52 facing each other with a separator 53 betweenthem, the cathode pellet 52 is fitted into a cathode ring 55, as shownin FIG. 2. In the coin-shaped lithium manganese battery of thisstructure, hereinafter referred to as Conventional Example 2, thecathode ring 55 limits the change in volume of the cathode pellet 52 inthe course of discharge, to the vertical direction. Thus, the reductionin the total height of the anode pellet 51 and the cathode pellet 52during discharge is prevented.

In addition, another coin-shaped lithium manganese battery, hereinafterreferred to Conventional Example 3, has been proposed. This coin-shapedlithium manganese battery of Conventional Example 3 has a thinnerbattery can 64 which contains therein an anode pellet 61 of greaterdiameter than that of Conventional Example 1 and a cathode pellet 62facing each other with a separator 63 between them, as shown in FIG. 3.

With above-described conventional coin-shaped lithium manganesebatteries, internal resistance R1 prior to discharge and internalresistance R2 after discharge of 80% of nominal capacity were measured.The results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                                     Internal                                                                              Internal                                           Nominal  Discharge Resistance                                                                            Resistance                               Battery   Capacity Capacity  R1      R2                                       ______________________________________                                        Conventional                                                                            190 mAh  191 mAh   8 Ω                                                                             63 Ω                               Example 1                                                                     Conventional                                                                            190 mAh  193 mAh   8 Ω                                                                             20 Ω                               Example 2                                                                     Conventional                                                                            220 mAh  228 mAh   7 Ω                                                                             101 Ω                              Example 3                                                                     ______________________________________                                    

The coin-shaped lithium manganese battery of Conventional Example 1 hasa problem such that internal resistance increases in the latter half ofdischarge, as shown in Table 1. This increase in internal resistance inthe latter half of discharge indicates that the battery in practical usehas its discharge voltage reduced when heavy load which deterioratesbattery properties particularly at low temperatures is imposed on thebattery. Consequently, equipments using this battery may not operatenormally.

The coin-shaped lithium manganese battery of Conventional Example 2requires the cathode ring 55, and thus has problems of increase in thenumber of component parts and hence increase in costs.

The coin-shaped lithium manganese battery of Conventional Example 3,though having a greater capacity than in Conventional Example 1, hasproblems of larger increase in internal resistance in the latter half ofdischarge and deterioration of performance by heavy load particularly atlow temperatures.

OBJECT AND SUMMARY OF THE INVENTION

In view of the above-described status of the art, it is an object of thepresent invention to provide a coin-shaped lithium battery which iscapable of maintaining satisfactory contact of anode with cathode evenin the latter half of discharge without causing an increase in thenumber of component parts, and exhibiting satisfactory dischargeproperties as well as low costs.

To achieve the above-mentioned object, the present inventors have founda structure in which the central part of at least an anode pellet and acathode pellet is swollen to maintain satisfactory contact of anode withcathode in the latter half of discharge by utilizing elasticity of thebattery can.

The present invention has been completed in view of the above. Accordingto the present invention, there is provided a coin-shaped lithiumbattery having a battery can containing therein an anode pellet composedof lithium or lithium alloy and a cathode pellet facing each other witha separator provided between them, at least one of the anode pellet andthe cathode pellet having its central part swollen to form a curvedsurface, the battery can being elastically deformed to be aligned withthe curved surface.

To elastically deform the battery can, one or both sides of the anodepellet may be made curved surfaces, or one or both sides of the cathodepellet may be made curved surfaces. Also, one side of the anode pelletand one side of the cathode pellet, both sides of the anode pellet andone side of the cathode pellet, one side of the anode pellet and bothsides of the cathode pellet, or both sides of the anode pellet and bothsides of the cathode pellet may be made curved surfaces.

The curved surface in this case may be spherical or paraboloidal.

With the curved surface, it is preferred that the total height of theanode pellet and the cathode pellet in the outer rim part is smallerthan the total height in the central part by 4 to 12%.

Thus, as the coin-shaped lithium battery of the present invention hasits battery can elastically deformed to be aligned with the curvedsurface produced by swelling the central part of at least one of theanode pellet and the cathode pellet, satisfactory contact of anode withcathode can be maintained by the elasticity. Therefore, the increase ininternal resistance at the end of discharge can be effectivelyprevented.

Also, as the coin-shaped lithium battery of the present invention hasthe total height of the anode pellet and cathode pellet in the outer rimpart smaller than the total height in the central part by 4 to 12%, thedischarge capacity can be secured and the increase in internalresistance in the latter half of discharge can be effectively prevented.

Thus, according to the present invention, the coin-shaped lithiumbattery which is capable of maintaining satisfactory contact of anodewith cathode without increasing the number of component parts, andexhibiting satisfactory discharge properties, high capacity and lowcosts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing essential portions of thestructure of a coin-shaped lithium manganese battery in ConventionalExample 1.

FIG. 2 is a cross-sectional view showing essential portions of thestructure of a coin-shaped lithium manganese battery in ConventionalExample 2.

FIG. 3 is a cross-sectional view showing essential portions of thestructure of a coin-shaped lithium manganese battery in ConventionalExample 3.

FIG. 4 is a cross-sectional view showing essential portions of thestructure of a coin-shaped lithium manganese battery in Example 1 of thepresent invention.

FIG. 5 is an exploded view of the coin-shaped lithium manganese batteryof Example 1.

FIG. 6 is a cross-sectional view showing essential portions of thestructure of a coin-shaped lithium manganese battery in Example 2 of thepresent invention.

FIG. 7 is an exploded view of the coin-shaped lithium manganese batteryof Example 2.

FIG. 8 is a cross-sectional view showing essential portions of thestructure of a coin-shaped lithium manganese battery in Example 3 of thepresent invention.

FIG. 9 is an exploded view of the coin-shaped lithium manganese batteryof Example 3.

FIG. 10 is a view showing results of measuring discharge capacity andinternal resistance after discharge of the coin-shaped lithium manganesebattery of Example 1, with the height of the anode pellet being changedbetween the central part and the outer rim part by 0 to 14%.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the coin-shaped lithium battery according tothe present invention will now be described with reference to theattached drawings.

EXAMPLE 1

FIG. 4 shows a coin-shaped lithium manganese battery of Example 1. Inthis Example 1, an anode pellet 11 has its one side curved, to have itsouter rim part thinner than the central part by 0.22 mm.

The coin-shaped lithium manganese battery of Example 1 is constructed asshown in an exploded view of FIG. 5. That is, the anode pellet 11 ismade up of molded metallic lithium having its one side 11A curved tohave the thickness t₁₀ in the outer rim part smaller than the thicknesst₁₁ in the central part by 0.22 mm, and having the other side on ananode cap 12 flattened. Then, a separator 15 is put on the anode pellet11, with an insulation sealing gasket 17 fitted therein. An electrolyteand a normal parallel disc-shaped cathode pellet 13 are entered, coveredwith a cathode can 14. The anode cap 12 and the cathode can 14 arecaulked with the insulation sealing gasket 17 provided between them, sothat the anode cap 12 is elastically deformed to substantially flattenthe inner surface of the anode pellet 11, as shown in FIG. 4.

EXAMPLE 2

FIG. 6 shows a coin-shaped lithium manganese battery of Example 2. Inthis Example 2, a cathode pellet 23 has its both sides 23A, 23B curved,to have the peripheral part thinner than the central part by 0.22 mm.

The coin-shaped lithium manganese battery of Example 2 is constructed asshown in an exploded view of FIG. 7. That is, an anode pellet 21 isformed in a normal parallel disc shape of molded metallic lithium on ananode cap 22. Then, a separator 25 is put on the anode pellet 21, withan insulation sealing gasket 27 fitted therein. An electrolyte and thecathode pellet 23 having the thickness t₂₁ in the outer rim part smallerthan the thickness t_(2O) in the central part by 0.22 mm are entered,covered with a cathode can 24. The anode cap 22 and the cathode can 24are caulked with the insulation sealing gasket 27 provided between them,so that the anode cap 22 and the cathode can 24 are elastically deformedto be aligned with curved surface of the cathode pellet 23, as shown inFIG. 8.

EXAMPLE 3

FIG. 8 shows a coin-shaped lithium manganese battery of Example 3. Inthis Example 3, an anode pellet 31 has its one side 31A curved and acathode pellet 33 has its both sides 33A, 33B curved, to have the outerrim part thinner than the central part by 0.22 mm.

The coin-shaped lithium manganese battery of Example 3 is constructed asshown in an exploded view of FIG. 9. That is, the anode pellet 31 ismade up of molded metallic lithium having its one side 31A curved tohave the thickness t₃₁ in the outer rim part smaller than the thicknesst₃₀ in the central part, and having the other side on an anode cap 32flattened. Then, a separator 35 is put on the anode pellet 31, with aninsulation sealing gasket 37 fitted therein. An electrolyte and thecathode pellet 33 having the thickness t₄₁ in the outer rim part smallerthan the thickness t₄₀ in the central part are entered, covered with acathode can 34. The anode cap 32 and the cathode can 34 are caulked withthe insulation sealing gasket 37 provided between them, so that theanode cap 32 and the cathode can 34 are elastically deformed to bealigned with curved surfaces of the anode pellet 31 and the cathodepellet 33, respectively, as shown in FIG. 8. The sum (t₃₀ +t₄₀) of thethicknesses t₃₀ and t₄₀ in the central parts of the anode pellet 31 andthe cathode pellet 33 is greater than the sum (t₃₁ +t₄₁) of thethicknesses t₃₁ and t₄₁ in the outer rim parts by 0.22 mm.

With the coin-shaped lithium manganese batteries of Example 1, Example 2and Example 3, internal resistance R1 prior to discharge and internalresistance R2 after 80% discharge of nominal capacity 220 mAh weremeasured. The results are shown in Table 2 in comparison withConventional Example 3.

                  TABLE 2                                                         ______________________________________                                                  Discharge  Internal    Internal                                     Battery   Capacity   Resistance R1                                                                             Resistance R2                                ______________________________________                                        Example 1 228 mAh    7 Ω   22 Ω                                   Example 2 227 mAh    7 Ω   20 Ω                                   Example 3 228 mAh    7 Ω   19 Ω                                   Conventional                                                                            228 mAh    7 Ω   101 Ω                                  Example 3                                                                     ______________________________________                                    

As is clear from the measurement results shown in Table 2, the increasein internal resistance at the end of discharge was effectively preventedwith the coin-shaped lithium manganese batteries of Examples 1 to 3,compared with the coin-shaped lithium manganese battery of ConventionalExample 3. Thus, the increase in internal resistance at the end ofdischarge can be effectively prevented with any of the batteries ofExample 1 in which the central part of the anode pellet 11 is swollen toform a curved surface, Example 2 in which the central part of thecathode pellet 23 is swollen to form a curved surface, and Example 3 inwhich the central parts of the anode pellet 31 and the cathode pellet 33are swollen to form curved surfaces. That is, in the present invention,at least one of the anode pellet and the cathode pellet has its centralpart swollen into a curved surface, to elastically deform the batterycan to be along with the curved surface. Thus, the increase in internalresistance at the end of discharge can be effectively prevented.

In Example 1 in which the central part of the anode pellet 11 is swolleninto a curved surface, discharge capacity and internal resistance R2after 80% discharge of nominal capacity 220 mAh were measured with theheight of the anode pellet 11 changed between the central part and theouter rim part by a rate of 0 to 14%. The results are shown in Table 3and FIG. 10.

                  TABLE 3                                                         ______________________________________                                        Rate of Difference                                                                            Discharge  Internal                                           in Height       Capacity   Resistance R2                                      ______________________________________                                        0%              228 mAh    102 Ω                                        2%              227 mAh    42 Ω                                         4%              228 mAh    28 Ω                                         6%              226 mAh    20 Ω                                         8%              223 mAh    20 Ω                                         10%             219 mAh    21 Ω                                         12%             216 mAh    27 Ω                                         14%             211 mAh    35 Ω                                         ______________________________________                                    

As is clear from Table 3 and FIG. 10, with the rate of difference inheight between the central part and the outer rim part varying in arange of 4 to 12%, discharge capacity can be secured and the increase ininternal resistance in the latter half of discharge can be effectivelyprevented.

What is claimed is:
 1. A coin-shaped lithium battery comprising:abattery can containing therein an anode pellet composed of lithium orlithium alloy and a cathode pellet which are arranged to face each otherwith a separator provided between the pellets, said separator having asubstantially planar configuration prior to assembly; at least one ofthe anode pellet and the cathode pellet having its central part swollento form a curved surface, each said curved surface being spherical orparaboloidal; and the battery can being deformed to be aligned with thecurved surface, whereby a lithium battery whose internal resistance asmeasured after 80% discharge of nominal capacity of about 220 mAh isless than or equal to 28 Ω is provided.
 2. The coin-shaped lithiumbattery as claimed in claim 1, wherein total height of the anode pelletand the cathode pellet in outer rim part is smaller than total height incentral part by 4 to 12%.
 3. The coin-shaped lithium battery as claimedin claim 1, wherein active material of the cathode pellet is composedmainly of manganese dioxide.
 4. The coin-shaped lithium battery asclaimed in claim 2, wherein active material of the cathode pellet iscomposed mainly of manganese dioxide.
 5. The coin-shaped lithium batteryas claimed in claim 1, wherein the anode pellet has at least one curvedsurface.
 6. The coin-shaped lithium battery as defined in claim 1,wherein the anode pellet has a pair of curved surfaces.
 7. Thecoin-shaped lithium battery as defined in claim 1, wherein the anodepellet includes at least one curved surface and the cathode pelletincludes a pair of opposed curved surfaces.